How effective are condoms in preventing AIDS?
Actually, they are not very effective at all. Undoubtedly because a condom retains the bulk of the discharge of semen, the rate is cut down. To become infected with any disease, two things are relevant in terms of the offending, invasive agent. One is the presence of some of those viruses, or germs, itself. The other is the quantity of that agent entering the body. If there are only a few bugs, the average human will kill them off. If there’s a very heavy dose, the chance for an infection succeeding is greater. Therefore, by cutting down on the volume of semen, a condom certainly reduces the chance for AIDS. But AIDS viruses do get through the condom.
Why is this?
Sperm have a diameter of 50 microns. Naturally occurring holes in the wall of a latex condom have a diameter of 1.0 microns. The HIV retrovirus which causes AIDS has a diameter of 0.1 microns. In effect, this would be comparing perhaps an ant crawling on a basketball. AIDS viruses swim freely through the holes in the condom. That is a fact that should be widely publicized.
(Source: Chapter 35 "Contraception," in Dr. and Mrs. J.C. Willke's book Why Can't We Love Them Both?) (Online edition here.)